In a large mixing bowl, beat the butter, brown sugar, and granulated sugar with a mixer until smooth and creamy. Add the egg and vanilla and mix together. In a separate bowl, stir together the flour, cornstarch, baking soda, and salt. Slowly mix in the flour mixture into the wet mixture until well combined. Stir in the white chocolate chips and dried cranberries. Chill the dough for at least 2 hours. Preheat your oven to 350 degrees F and line a large baking sheet or two with parchment paper. Roll the dough into 1 inch balls and place on the baking sheet(s) 1-2 inches apart. Bake for 8-10 minutes or until golden around the edges. Allow the cookie to cook on the sheet for 5 minutes then transfer to a wire rack to cook. Serve or store in an airtight container.